<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<div id="app">
    <span v-once>{{msg}}</span>
    <span>{{msg}}</span>
</div>
</body>
<script src="./js/vue.js"></script>
<script>
    new Vue({
        el:'#app',
        data:{
            msg:0
        },
        mounted(){
            setInterval(()=>{
                this.msg++
            },1000)
        }
    })
</script>
</html>